1. Field of the Invention
The present invention relates generally to optical information processing systems, and more particularly pertains to a write with light optical notching filter system in which an optical spatial filter therein is formed by a liquid crystal spatial light modulator (SLM) which is written onto with light.
2. Discussion of the Prior Art
In selective frequency filtering, there are significant advantages to performing the filtering function optically with spatial filters instead of by the use of a more common electronic filter, particularly if a number of filters are desired. Optical spatial filtering is typically accomplished in the prior art with filtering systems having either a single stage, or multiple recursive stages as disclosed and taught, for example, by U.S. Pat. Nos. 4,522,466 and 4,645,300. In single stage prior art systems using a programmable spatial filter, the maximum filter attenuation is approximately 25 db. In multiple recursive stage systems, the programmable spatial filter (PSF) stages are effectively cascaded by means of recursions, and the attenuation achieved thereby can be increased as a function of the number of stages or recursions.
Such programmable spatial filter systems usually incorporate a source of collimated coherent light, and the light beam therefrom is modulated by a suitable transducer, which is followed by a convex lens, an optical spatial filter, which can be a programmable spatial filter, a second convex lens, and a detection means. In such a system, light from the source passing through the transducer is modulated and then forms a diffraction pattern (Fourier transform) at the focal plane of the first lens. The second lens provides the inverse Fourier transform of the diffraction pattern that passes through the spatial filter, resulting in a spatial frequency filtered representation of the modulated input light signal. The light signal output of the second lens contains all of the spatial frequencies appearing in the Fourier plane minus those selectively filtered out by the programmable spatial filter. Changing the spatial frequency distribution of the programmable spatial filter in the Fourier plane results in an inverse transform image with a corresponding resolution/high frequency, contrast/low spatial frequency and phase response.
In an optical, filtering system for RF signals, RF input signals are fed into an acousto-optic modulator to modulate a laser beam. The modulated output beam is passed through an optical Fourier transform lens to produce a spatial frequency distribution at its back focal plane. This signal contains a one-for-one spatial and temporal correspondence with the RF frequency distribution. The transformed beam is then directed through the spatial filter which is also located at the back focal plane of the transform lens. A programmable spatial filter is used for the filtering function, and the optical transmission from point-to-point is controlled by the spatial filter such that some spatial frequencies are blocked and others are passed in accordance with the programmed notch frequencies. Optical spatial frequencies passing through the spatial filter consist of the laser optical carrier frequency modulated with the RF frequency. An optical inverse Fourier transform lens images the filtered beam, and the filtered beam is then passed to an optical mixer where it is mixed with a local oscillator reference beam. Optically combining the modulated laser beam with the local oscillator beam and directing the sum onto a square-law photodetector results in the generation of the difference frequency by a heterodyning action. The electrical output of the photodetector is amplified and initially filtered, and can then be subjected to conventional post processing.
The prior art has also addressed the concept of adaptive filtering, or the electronic programming of a filter in accordance with a time varying criteria. The concept of a filter matched to the spectrum of the desired signal theoretically should provide optimum filtering. In actual practical situations, particularly when detecting spread spectrum signals contaminated with higher level narrow band signals, a narrow band (notch) filter within the passband of the wide band spread spectrum signal can reduce or eliminate the high level signal from the spread spectrum signal.